As is known in the art, there are various applications where air is transported through a tube or pipe for sampling or measurement purposes. For example, an air quality measurement system may have remotely located sensors instead of at the sensed environment. In addition, a sensor may be used to sense several locations. For such systems, multiple tubes may be used to bring air samples from multiple locations to a centralized sensor(s). Centrally located air switches and/or solenoid valves may be used in these approaches to sequentially switch the air from these locations through the different tubes to the sensor to measure the air from multiple remote locations. These octopus-like or star-configured systems use considerable amounts of tubing. Another multiple-location sampling system known as a networked air sampling system uses a central ‘backbone’ tube with branches extending to various locations. Air solenoids can be remotely located proximate the multiple sampling locations. Air sampling systems can include remote and/or multiple-location air sampling through a tube or pipe for sampling locations in a building, outdoor air or ambient sampling, and sampling in smokestacks and exhaust air stacks. An exemplary air sampling system is described in U.S. Pat. No. 6,125,710, which is incorporated herein by reference.
As known in the art, air sampling systems can use various types of tubing to transmit air samples or ‘packets’ to the appropriate sensor. One type of tubing is TEFLON tubing. However, TEFLON tubing is relatively expensive and has certain undesirable particle transport characteristics, such that it is a poor conductor and tends to establish a charge as an air sample passes through a tube of such material resulting in enhanced electrostatic deposition of particulate matter from the flow stream. Low or High Density Polyethylene (LDPE or HDPE) tubing, which is less expensive than TEFLON tubing, has been used with limited success. Although good for indoor air quality CO2 sensing, the LDPE or HDPE tubing absorbs and desorbs volatile organic compounds (VOCs) leading to inaccurate sensing results. This type of tubing is also poor for particle sensing applications since the plastic is an electrically poor conductor and can hold a charge resulting in relatively poor transport properties as a result of electrostatic deposition.
Some types of plastic tubing can be used for transporting particles. For example, one type of plastic tubing is known as “Bev-A-Line XX” tubing made by Thermoplastic Processes, Inc. of Stirling, N.J., can be used to perform air sampling with particle transport efficiencies that are an improvement over that possible with polyethylene tubing. However, “Bev-A-Line XX” tubing is quite expensive and absorbs VOCs. Similarly, silicone tubing is sometimes impregnated with large amounts of carbon black (a powdered form of highly dispersed elemental carbon) to create conductive silicone tubing for transporting particles. For example, TSI, Inc., manufactures various sized conductive silicone tubing such as their part number 3001789, which has an inner diameter of 0.31 inches and an outer diameter of 0.375 inches. This tubing is typically used in aerosol research to transport aerosol particles to portable instruments. However, conductive silicone tubing also absorbs and desorbs VOCs. Also silicone specifically attacks and compromises the operation of metal oxide semiconductor TVOC (Total Volatile Organic Compound) sensors of the type that are commonly used to measure the level of VOCs in air samples.
While certain metal tubing may have desirable properties for transporting air samples, known metal tubing options may have certain drawbacks. For example, some metal tubing is rigid rendering it quite expensive to install, because of the labor involved with that process. While other metal tubing may be deformable so as to facilitate installation, the metal characteristics are not well suited for air sampling applications. One known tubing manufactured by Synflex of Mantua Ohio, a division of Saint-Gobain Performance Plastics, includes an aluminum-lined polyethylene tube (Type 1300 Synflex, formerly known as Dekabon) to provide a stronger plastic tube with a higher burst resistance and pressure rating for high pressure pneumatic applications. The internal aluminum liner is also coated with an adhesive to help attach the aluminum inner tube together with the outer plastic jacket. It also has a plastic coating on the inner portion of the tube for added chemical resistance. However, such a tubing configuration is undesirable for use as an air-sampling medium. The inner coating attracts and traps particles and absorbs VOCs. In addition, even if the coating was not used the aluminum is reactive with many indoor contaminants. Due to its reactive nature, the aluminum tubing would not give accurate and reliable performance as an air sampling tubing. Further, the aluminum surface has an affinity to oxidize over time as it is exposed to ambient air conditions. The surface oxidation increases the roughness of the inside of the tube and can result in the release of particulate matter in the form of aluminum oxide, which can have a non-negligible impact on a given concentration of particulate matter being sampled via transport through the tubing.